This invention is related to the power generation industry and, more particularly, to the field of power generators and coils for the same.
In the power generation industry, high voltage coils are often used in stators of power generators. These coils often include one or more layers of groundwall insulation. This groundwall insulation is often formed of glass backed mica paper (GBMP) tape impregnated with an epoxy resin. Both electrical and mechanical requirements for these high voltage coils, however, can be difficult to meet with the conventional groundwall insulation systems. The end turn support and bracing components subjects the coil insulation to very high compressive forces. The straight or medial portions of the coil are less subject to such high compressive forces. The conventional epoxy resin impregnated GBMP tape, for example, is not suitable for compressive forces exceeding 3000 pounds per square inch (psi) at elevated temperatures. Separation of mica platelets can occur, for example, at high compressive forces. The small mica platelets are unable to maintain physical contact sufficiently to provide good mechanical strength and good dielectric strength.
Although resin rich (RR) tapes such as shown in U.S. Pat. No. 4,112,183 by Smith titled xe2x80x9cFlexible Resin Rich Epoxide-Mica Winding Tape Insulation Containing Organo-Tin Catalysts,xe2x80x9d U.S. Pat. No. 4,296,018 by Smith titled xe2x80x9cCatechol or Pyrogallol Containing Flexible Insulating Tape Having Low Gel Time,xe2x80x9d U.S. Pat. No. 4,224,541 by Smith titled xe2x80x9cFluid Solventless Epoxy-Anhydride Compositions Containing Metal Acetylacetonate Accelerators And Organic Carboxylic Acid Co-Accelerators For Use On An Electrical Member,xe2x80x9d and U.S. Pat. No. 4,704,322 by Roberts titled xe2x80x9cResin Rich Mica Tape,xe2x80x9d have been developed over the years for application to electrical members, such tapes are conventionally applied as a groundwall insulation and are rigid or stiff, bulky, and cause various problems as such a groundwall insulation. Even various curing and tape compositions have been attempted to strengthen such tapes or attempt to address some of the compressive creep problems with little or no success.
In view of the foregoing, the present invention advantageously provides a method and apparatus for increasing compressive creep capabilities of high voltage coil insulation. The present invention also advantageously provides a method and apparatus for increasing compressive creep capabilities of high voltage coil insulation without any change in the materials used in the groundwall insulation. The present invention further advantageously provides a method and apparatus for increasing compressive creep capabilities of high voltage coil insulation which provides significant protection to the groundwall insulation and substantially reduces instances of fractures to the groundwall insulation.
More particularly, the present invention provides a method for increasing compressive creep capabilities of high voltage coil insulation. The method preferably includes applying at least one layer of a first tape as a groundwall insulation to substantially the entire length of a high voltage coil. The first tape preferably has a first predetermined amount of resin therein. The method also includes curing the groundwall insulation for a first predetermined time period, applying at least one layer of a second tape as a protective armor overlying the groundwall insulation only to end portions of the high voltage coil after the first predetermined time period. The second tape preferably includes substantially more resin than the first tape. The method further includes curing the protective armor for a second predetermined time period.
The present invention also provides an apparatus to increase compressive creep capabilities of high voltage coil insulation which preferably includes a high voltage coil having a predetermined length, at least one layer of resin impregnated tape overlying the high voltage coil along substantially the entire predetermined length thereof and defining groundwall insulation, and at least one layer of resin rich (RR) tape overlying the groundwall insulation only along end portions thereof to define a protective armor. The RR tape of the protective armor being impregnated with substantially more resin than the resin impregnated tape of the groundwall insulation.